1. Field of the Invention
The present invention relates to a humanoid robot, and more particularly, to a foot structure for a humanoid robot capable of effectively keeping its balance in a pause state and/or action state.
2. Description of the Related Art
Robots are generally utilized for factory automation and play an important role in non-human environments. Recently, humanoid robot systems are being actively developed because robots capable of performing in a human environment such as offices, homes, or hospitals are desired. Accordingly, some humanoid robots have been developed.
One of most important conditions of a humanoid robot is to guarantee an effective interaction between a foot of the robot and the ground. If there is no effective interaction between the foot of the robot and the ground, a shock is generated when a heel of the robot's foot touches on the ground, and is then transferred to the robot's foot. Furthermore, the shock is transferred to the robot's body through an ankle joint. Due to such a shock, the dynamic equilibrium of the robot may be disturbed and an unstable gait of the robot may be caused. In addition, a vibration of the foot resulted from the shock is transferred to the body of the robot, thereby lowering a control stability of the robot.
Another problem resulted from the unstable interaction between the robot's foot and the ground occurs when the robot passes an area where small obstacles exist on the ground or a surface of the ground is not flat, even though the robot may use a vision system for assessing the surroundings. In this case, the robot's foot does not adequately step on the ground, which deteriorates the stability and spontaneity of the gait of the robot.
In order to solve the above problems, several robot's foot mechanisms capable of absorbing a shock during walking have been proposed. Among them are the foot structures disclosed in the article “The development of Honda humanoid robot” by K. Hirai et al., Proceedings of the 1998 IEEE International Conference on Robotics & Automation, Leuven, Belgium, May 1998, and U.S. Pat. No. 6,377,014, “Legged walking robot” issued to Gomi et al., on Jun. 23, 2002.
FIG. 1 is a cross-sectional view illustrating a structure of a conventional robot foot disclosed by K. Hirai et al.
Referring to FIG. 1, a foot 10 includes a sole 14 made of rubber and a bushing 12 having a guide shape and made of rubber, thereby constructing a mechanism capable of absorbing a shock. The foot 10 of the robot is elastically deformed in a vertical direction by a ground force applied from the sole 14. With the above construction, the transfer of an impact load is reduced, since the foot serves mechanically as a low-pass filter to prevent the vibration of a leg so that the leg can be smoothly controlled.
However, the foot 10 does not provide a stable walking mechanism. For this, the rubber used for the sole 14 and the bushing 12 has to have high elasticity in order to secure the stability of the upper portion of the robot relative to the sole 14 so that a tight contact between the sole 14 and the ground can be achieved.
Consequently, the total stiffness of the foot is increased and becomes approximately a constant regardless of a value of the ground force. Therefore, this type of shock-absorbing apparatus has a disadvantage in that it cannot effectively attenuate the vibration generated after a shock is applied to the heel of the robot.
In order to overcome the above problem, U.S. Pat. No. 6,377,014 teaches a robot foot having a sole consisting of a first elastic portion and a second elastic portion, and the second elastic portion having a heel portion consisting of a flat central portion of a relatively thin thickness and a plurality of projections of a relatively thick thickness which are spaced apart from each other at regular intervals.
Meanwhile, a structure of a humanoid robot employing a six-axis force sensor that makes the robot walk stably on a rough and flat terrain is disclosed in the article “Design of advanced leg module for humanoid robotics project of METI,” by Kenji Kaneko et al., Proceedings of the 2002 IEEE International Conference on Robotics & Automation, Washington, D.C., May 2002.
FIG. 2 is an explanatory view depicting a foot structure of the conventional robot of using a six-axis force sensor.
As shown in FIG. 2, a foot 20 of the robot includes a six-axis force sensor 22, a plurality of rubber bushes 24, and a sole plate 26. It is very important to control a torque of the sole in order to make the robot walk stably on rough terrain. Since the plurality of bushes 24 serve as a compliance element with a roll axis and a pitch axis, the rotational deformation of the plurality of bushes 24 is controlled by the 6-axis force sensor 22, thereby controlling the torque applied to the sole of the robot.
In addition, WAF-2 (Waseda Anthropomorphic Foot No. 2), an advanced foot structure for a humanoid robot, and a controlling system for the WAF-2 are disclosed in the article “Experimental development of a foot mechanism with shock absorbing material for acquisition of landing surface position information and stabilization of dynamic biped walking,” by Jinichi Yamaguchi et al., IEEE International Conference on Robotics & Automation, pp 2892-2899 (1995).
FIG. 3A is a view depicting a structure 30 of WAF-2.
As shown in FIG. 3A, the WAF-2 structure 30 includes an upper foot plate 40, four potentiometers 36 placed on the upper foot plate 40, four wires 38 penetrating the upper foot plate 40, a plurality of upper stoppers 50 placed under the upper foot plate 40, a shock-absorbing member 44 positioned under the upper foot plate 40, a first acrylic plate 42 positioned between the upper foot plate 40 and the shock-absorbing member 44, a lower foot plate 62, a second acrylic plate 46 placed between the lower foot plate 62 and the shock-absorbing member 44, a plurality of lower stoppers 60 placed on the lower foot plate 62, and a plurality of spikes 64 formed under the lower foot plate 62.
FIG. 3B is a perspective view depicting one of the plurality of upper stoppers 50 placed under the upper foot plate 40 in FIG. 3A.
As shown in FIG. 3B, the upper stopper 50 consists of a duralumin 52, a urethane rubber 54, a Teflon resin 56, and a silicon foam 58.
As shown in FIGS. 3A and 3B, the WAF-2 structure 30 has a passive shock-absorbing mechanism achieved by the shock-absorbing member 44 placed between the upper foot plate 40 and the lower foot plate 62 and the plurality of upper stoppers 50 placed under the upper foot plate 40. In the case of a ground force having a nonlinear characteristic where the foot stiffness is gently increases with the ground force, such a structure can provide a shock-absorbing mechanism. In addition, this structure provides a stabilizing function by the upper stoppers 50 and the lower stoppers 60 placed on the spikes 64, and the upper foot plate 40 and the lower foot plate 62, respectively.
A two-legged robot using this structure can easily operate on a surface having variations of a few millimeters in a vertical or horizontal direction or at an inclination angle of about 1°.
However, the humanoid robots employing the WAF-2 or WAF-3 structure have several drawbacks.
The humanoid robot has a nonlinear stiffness characteristic similar to a human being only at a nominal load corresponding to a robot having a weight of about 110 kg and within a range of 6 mm. In other words, the maximum stiffness of the foot is equal to 91.7 N/mm under a restricted condition. If the maximum ground force is above the value, the upper foot plate is completely contacted with the lower foot plate. Thus, an average stiffness of the foot is determined by a metal portion having high stiffness and low damping.
In addition, since the WAF-2 or WAF-3 structure utilizes a shock-absorbing material, such as a yellow memory foam M-36, the elastic characteristic varies. A heater is mounted in the foot structure to maintain the temperature in about 40° C., so as to prevent variations of the elastic characteristic. Thus, the foot structure is heavy and very complicated.
Finally, since the WAF-2 or WAF-3 structure has no toe joints, the robot cannot secure a tight contact between the ground and the foot in a push-off motion during walking. Therefore, the robot cannot secure a sufficient propelling force required for quick walk, run, carrying a load, or the like.